Aluminum-and magnesium-based molten ceramic grains

ABSTRACT

Molten ceramic grains are intended, for example, for applications involving abrading tools, having the following average chemical weight composition, expressed in weight percent based on oxide content: Al 2 O 3 : 93% to 97.5%; MgO: 2.2 to 6.5%; SiO 2 : &lt;0.1%; other impurities: &lt;0.4%.

BACKGROUND OF THE INVENTION

The invention relates to alumina-based ceramic grains obtained byfusion.

Aluminous ceramic grains are useful, among other applications, for themanufacture of abrasive tools. In general, abrasive tools are classifiedaccording to the method of forming the ceramic grains of which they arecomposed: free abrasives (grains used by spraying them or used insuspension, with no backing); coated abrasives (a cloth or paper typebacking coated with the grains, these being conventionally arranged inseveral layers); and bonded abrasives (bonded grains in the form ofgrinding wheels, sticks, etc.).

In the case of bonded abrasive tools, the abrasive grains are pressedwith an organic or glassy binder. Glassy binders generally consist ofoxides, essentially silicates. The bonded grains must themselves exhibitgood mechanical abrasion properties, in particular must have goodtoughness. They must also be able to be bonded strongly to the binder(interfacial strength).

At the present time, there are various families of ceramic grains thatallow all these applications to be covered with a variety ofperformance. In particular, two large families can be distinguished inwhich the grains are obtained by a sol-gel method or by fusion.

The sol-gel method, as described for example in EP 1 228 018 (U.S. Pat.No. 6,287,353), makes it possible to manufacture grains with a very finecrystalline structure, conventionally on a submicron scale, which givesthem excellent cutting effectiveness and a long life time. However, theproductivity of the sol-gel method is low and incurs high manufacturingcosts.

Fused grains, obtained by fusing the raw materials or “fused grains”,conventionally have much coarser crystalline structures and have a lowercutting effectiveness and a shorter life time. Fused grains containingmainly alumina are described, for example, in U.S. Pat. No. 4,157,898.The main advantage of these grains is their low manufacturing cost.

The composition of the grains is important, but the manufacturingprocess also plays a determining role on the performance. Thus, for agiven composition, a microstructure obtained by the sol-gel route, andoffering advantageous properties, cannot easily be obtained by fusion.

Table 1 below provides, for comparison, the results in a fracturingresistance test (test A), described in greater detail later on in thedescription, for two high-alumina abrasive grains of the prior art.These two grains are manufactured and sold by Saint-Gobain IndustrialCeramics. The white corundum grain is obtained by fusion and the Cerpassgrain by the sol-gel method. As table 1 shows, the chemical compositionsare very similar (the balance is alumina). However, white corundum givesa 119% result in test A, whereas Cerpass gives 375%.

TABLE 1 Test SiO₂ TiO₂ Na₂O MgO CaO Fe₂O₃ Cr₂O₃ A White <0.1% <0.05%0.27% <0.02% <0.02% 0.02% — 119 corundum Cerpass 0.061% 0.096% <0.03%0.009% 0.014% — 0.003% 375

FIGS. 1 and 2 appended hereto show, in cross section, white corundum andCerpass grains, respectively.

There is therefore a need for fused aluminous grains offering betterperformance both in terms of life time and cutting effectiveness thanthat of the current fused aluminous grains, but which can bemanufactured for a substantially lower cost than that of the aluminousgrains obtained by the sol-gel method.

SUMMARY OF THE INVENTION

The objective of the present invention is to satisfy this need.

According to the invention, this objective is achieved by means of fusedgrains having the following average chemical composition by weight, inpercentages by weight on the basis of the oxides:

Al₂O₃: 93% to 98.5%;

MgO: 2.2 to 6.5%;

SiO₂: <0.1%;

other impurities: <0.4%.

As will be seen later, such grains, manufactured by fusion, areinexpensive but nevertheless have a long life time and excellent cuttingeffectiveness.

Unless mentioned to the contrary, the percentages used in the presentapplication always refer to percentages by weight on the basis of theoxides.

According to other preferred features of the invention:

-   -   the minimum magnesia (MgO) content, as a percentage by weight on        the basis of the oxides, is 2.3%, preferably 2.45% and the        maximum MgO content, as a percentage by weight on the basis of        the oxides, is 4%, preferably 2.5%;    -   the maximum carbon content is 250 ppm, preferably 200 ppm; and    -   the maximum Na₂O content, as a percentage by weight on the basis        of the oxides, is 0.1%, preferably 0.05%.

The invention also relates to a process for manufacturing ceramicgrains, which comprises the following successive steps:

a) preparation of a mixture of raw materials having the followingaverage chemical composition by weight, as percentages by weight on thebasis of the oxides:

-   -   Al₂O₃: 93% to 97.5%;    -   MgO: 2.2 to 6.5%;    -   SiO₂: <0.1%;    -   other impurities: <0.4%;

b) fusion, in an electric arc furnace, by means of a short arc and witha melting energy before casting between 2000 and 2500 kWh per ton ofsaid mixture of raw materials, under defined reducing conditions so thatthe product obtained after the following step c) has a maximum carboncontent of 250 ppm;

c) casting and quench cooling, preferably so that the molten liquidsolidifies entirely in less than 3 minutes; and

d) grinding of the cooled product.

According to other preferred features of the process according to theinvention:

-   -   said mixture of raw materials also contains between 0.8 to 5.5        wt %, preferably 2.5 wt %, carbon and/or between 0.8 and 5.5 wt        %, preferably 2.5 wt %, aluminum metal chips;    -   said mixture of raw materials contains, as a percentage by        weight on the basis of the oxides, a minimum magnesia (MgO)        content of 2.3%, preferably 2.45%, and a maximum MgO content of        4%, preferably 2.5%;    -   the process includes, after step d), a calcination step in an        oxidizing atmosphere at a temperature above 1250° C., preferably        above 1350° C. and more preferably above 1400° C., the        calcination temperature preferably being maintained for a time        of at least 30 minutes; and    -   the process includes a final step, of screening the ground        grains and selecting the screened grains, the selected grains        preferably having a grit number of F50 or less according to the        FEPA Standard 42-GB-1984.

Finally, the invention relates to the use of the grains according to theinvention and/or of the grains obtained by means of the processaccording to the invention in abrasive products, preferably in bondedproducts or in coated products.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates white corundum, in cross-section.

FIG. 2 illustrates Cerpass grains, in cross-section.

FIG. 3 illustrates corundum crystals surrounded by a nonstoichiometricMgO—Al₂O₃ spinel phase.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following nonlimiting examples are given for the purpose ofillustrating the invention.

The products given as examples were prepared from an initial mixturecontaining the following raw materials:

-   -   alumina of the AR75 type, sold by Pechiney, containing on        average 99% Al₂O₃;    -   NedMag® magnesia containing about 98% MgO.

Silica and sodium oxide are known to be deleterious and their respectivecontents must be limited to trace amounts (<0.1%) introduced by way ofimpurities into the raw materials. This is because the presence ofsilica results in the formation of a glassy phase whose influence on theabrasive properties and on the hardness of the grain is deleterious. Thepresence of sodium oxide, even with low contents, results in theformation of beta-alumina. Now, this crystal form of alumina diminishesthe abrasive properties of the grains.

The contents of the other impurities, such as CaO, TiO₂, Fe₂O₃ or Cr₂O₃must not exceed 0.3%.

The initial mixture was melted in a conventional melting process usingan arc furnace in reducing medium, with the addition of 2.5 wt % carbon,for example petroleum coke, pitch or coal, and 2.5 wt % aluminum metalchips. The electric arc was short and the energy before casting wasbetween 2000 and 2500 kWh per tonne of the initial mixture of rawmaterials. The voltage needed to obtain a short arc depends on manyparameters, which are well controlled by those skilled in the art, suchas the size of the furnace, the number of electrodes and the size of theelectrodes.

The bath of molten raw materials was then rapidly cooled, in order topromote the formation of fine oriented structures, by means of castingdevices in which the material is cast between thin metal plates, such asthe devices presented in patent U.S. Pat. No. 3,993,119.

The cooled molten material or “crude” was then ground, for example inroll grinders, then screened and classified in series of particle sizedistributions (“number” or “grit”) according to the FEPA (FédérationEuropéene des Fabricants de Produits Abrasifs) Standard 42-GB-1984.

According to the invention, to improve the impact strength and theabrasion performance of the grains obtained, they were then subjected toa heat treatment step. Preferably, the heat treatment comprises acalcination in an oxidizing atmosphere, preferably in air, at atemperature above 1250° C., preferably above 1350° C. and morepreferably above 1400° C., for a time of at least 2 minutes, preferablyat least 5 minutes and more preferably at least 30 minutes.

For the examples, the grains were calcined in air at varioustemperatures for variable times. These operating parameters are given inTable 2 below.

The chemical composition of the products obtained is given in Table 2.This is an average chemical composition, given in percentage by weight.

The chemical composition, excluding the carbon content, was obtained byX-ray fluorescence.

The carbon content, which reflects the oxidation-reduction state, wasmeasured by infrared absorption. To do this, the specimen was milled,after magnetic separation, in a carbon-free milling jar (for examplemade of fused alumina-zirconia-silica) until a powder passing through a160 μm screen was obtained. The specimen thus prepared was analyzedusing a LECO® model CS300 instrument.

The control grain was a fused aluminous grain produced under reducingconditions and sold under the name 32AII by Saint-Gobain IndustrialCeramics. Its typical chemical composition comprises 99.4% Al₂O₃, 0.4%TiO₂, less than 0.1% Na₂O and less than 0.02% SiO₂.

To characterize their mechanical properties, the grains were subjectedto a fracturing resistance test (test A). The aim of this test was todetermine the fraction of surviving grains of a given particle size cutafter a milling operation in a steel milling jar.

An AUREC type T100 rotary mill was used, which rotated a hollowcylindrical jar containing the grains and also a ring and a disc. Thejar was made of a Z200C12 grade steel and had an inside diameter of 140mm and a height of 18 mm. The disc was cylindrical and hollow (diameter75 mm, height 46 mm and wall thickness 10 mm). The disc and the ringwere made of steel of the same grade as the jar.

The grains were firstly screened and classified according to thefollowing fractions to be tested:

-   -   710/850 μm, to represent the grain of F24 grit number;    -   500/600 μm, to represent the grain of F36 grit number;    -   300/355 μm, to represent the grain of F54 grit number;    -   250/300 μm, to represent the grain of F60 grit number;    -   180/212 μm, to represent the grain of F80 grit number; and    -   106/125 μm, to represent the grain of F120 grit number.

The grains were then de-ironed by magnetic separation. The jar wascleaned with compressed air before a specimen consisting of 25 grams ofgrains was introduced therein. The mill was then rotated at its nominalspeed (1400 rpm) for 5 seconds. The specimen was then extracted using abrush (No. 50). Its particle size distribution was then analyzed byintroducing it onto a series of screens using a ROTAP screening devicefor 5 minutes. The mass of grains not passing through the screen of 425μm opening was then measured. This mass, given as a percentage relativeto the remaining mass under the same conditions for the controlspecimen, corresponds to the result of test A.

It is considered that the value obtained in the test A must be greaterthan 190 (that is to say the mass of grains not passing through thescreen of 425 μm opening is at least 1.9 times greater than that of thecontrol), and preferably must be greater than 300 (that is to say themass of grains not passing through the screen of 425 μm opening is atleast 3 times greater than that of the control) in order for the effectto be sufficiently marked for these grains to be able to be used inabrasive products.

To evaluate the cutting effectiveness of the grains obtained, weemployed the following test B.

In this test, the preparation of the specimens was identical to that oftest A. The same apparatus and the same operating method were used.After screening the tested specimen, the entire specimen wasreintroduced into the mill for 145 seconds. The specimen was thenextracted using a hard brush and the iron content was measured by X-raydiffraction. This value, given as a percentage relative to the valueobtained in the same test for the control specimen, is the result oftest B.

It is considered that the value obtained in test B must be greater thanor equal to 70% in order for the cutting effectiveness to besatisfactory.

TABLE 2 Heat treatment Redox Grain (calcination) Composition in % byweight state size Temp. Time % % % Other C (Grit Test Test Ex. (° C.)(min) Al₂O₃ MgO SiO₂ % Na₂O Impur. (ppm) No.) A B 1 1400 35 >98.68 0.87<0.1 <0.05 <0.3 155 F24 162 89 2 1400 35 >98.2 1.35 <0.1 <0.05 <0.3 113F24 220 67 3 1300 60 >98 1.55 <0.1 <0.05 <0.3 74 F24 303 85 4 130060 >97.94 1.61 <0.1 <0.05 <0.3 83 F24 336 83 5 1400 120 >97.78 1.75 <0.10.07 <0.3 F24 377 88 6 1300 120 >97.08 2.47 <0.1 <0.05 <0.3 175 F24 36898 7 1400 5 >97.08 2.47 <0.1 <0.05 <0.3 165 F24 305 109 8 1400 30 >97.082.47 <0.1 <0.05 <0.3 165 F24 326 100 9 1400 45 >97.08 2.47 <0.1 <0.05<0.3 170 F24 382 93 10 1400 120 >97.08 2.47 <0.1 <0.05 <0.3 168 F24 34998 11 1400 600 >97.08 2.47 <0.1 <0.05 <0.3 165 F24 351 100 12 1400120 >96.85 2.68 <0.1 0.07 <0.3 F24 388 85 13 1400 35 >96.26 3.39 <0.1<0.05 <0.2 F24 303 14 1400 35 >96.26 3.39 <0.1 <0.05 <0.2 F54 158 151400 40 >96.26 3.39 <0.1 <0.05 <0.2 F80 106 16 1400 45 >96.03 3.6 <0.10.07 <0.2 315 F36 206 67 17 1400 50 >95.89 3.76 <0.1 <0.05 <0.2 220 F36275 70 18 1400 45 >95.89 3.76 <0.1 <0.05 <0.2 180 F36 193 105 19 1300120 >95.67 3.88 <0.1 <0.05 <0.3 90 F24 274 92 20 1400 5 >95.67 3.88 <0.1<0.05 <0.3 80 F24 282 94 21 1400 30 >95.67 3.88 <0.1 <0.05 <0.3 70 F24314 97 22 1400 120 >95.67 3.88 <0.1 <0.05 <0.3 72 F24 333 92 23 140045 >95.77 3.88 <0.1 <0.05 <0.2 F24 324 82 24 1400 45 >95.77 3.88 <0.1<0.05 <0.2 F60 134 68 25 1400 60 >95.69 3.95 <0.1 0.06 <0.2 F36 334 8326 1400 60 >95.69 3.95 <0.1 0.06 <0.2 F60 137 64 27 1400 60 >95.69 3.95<0.1 0.06 <0.2 F120 99 67 28 1400 20 >95.69 3.95 <0.1 0.06 <0.2 F60 9829 1300 20 >95.69 3.95 <0.1 0.06 <0.2 F60 97 30 1400 35 >95.11 4.54 <0.1<0.05 <0.2 75 F24 320 74 31 1400 35 >94.41 5.24 <0.1 <0.05 <0.2 F24 31673 32 1400 45 >93.53 6.12 <0.1 <0.05 <0.2 93 F24 309 71 33 140045 >93.06 6.59 <0.1 <0.05 <0.2 87 F24 305 62 34 1400 120 >92.54 7.01<0.1 <0.05 <0.3 106 F24 290 51

The examples in Table 2 demonstrate that an MgO content of greater than1.5% is necessary for the fracturing resistance of the grains to beimproved. Table 2 also indicates that an MgO content of greater than6.5% degrades the quality of the grain obtained, in particular itscutting effectiveness, but also the fracturing resistance is degradedtoo.

Preferably, the minimum MgO content is 2.2%, more preferably 2.45%.

Preferably, the maximum MgO content is 4%, more preferably 2.5%.

Complementary analyses (by scanning electron microscopy) for studyingthe crystalline phases showed that the products of the inventionconsisted of corundum crystals 1 (alpha-alumina) surrounded by anonstoichiometric MgO—Al₂O₃ spinel phase 2 (see appended FIG. 3). Themean size of the corundum crystals is 18 to 20 μm. Typically, 90% of thecrystals have a size of greater than 9 μm and 90% have a size of lessthan 27 μm. 100% of the crystals have a size of greater than 5 μm.

Comparison between Examples 13 and 14, or between Examples 23 and 24, orbetween Examples 26, 27 and 28, illustrates the influence of the grainsize. The grains have a better fracturing resistance and a bettercutting capacity the coarser they are, that is to say the smaller theirgrit number.

For highly demanding applications, for example when used in ahigh-pressure grinding tool, it is preferred to select the coarsestgrains, preferably the coarse grains having a grit number of F60 orlower, preferably a grit number of lower than F50, and more preferably agrit number of lower than F36.

The melting step of the process according to the invention underreducing conditions generates products having a low carbon content.Preferably, the process is carried out, using techniques that areconventional to those skilled in the art, so that the carbon content isless than 250 ppm, preferably less than 200 ppm and more preferably lessthan 180 ppm.

Advantageously, the carbon content of the grains according to theinvention makes them particularly suitable for use in bonded abrasives.

Of course, the present invention is not limited to the embodimentsdescribed and shown above, these being provided by way of illustrationbut implying no limitation.

1. Fused ceramic grains having the following average chemicalcomposition by weight, in percentages by weight on the basis of theoxides: Al₂O₃: 93% to 98.5%; MgO: 2.2 to 6.5%; SiO₂:<0.1%; otherimpurities:<0.4%, wherein the maximum carbon content is 200 ppm, andwherein the grains consist of corundum crystals surrounded by anonstoichiometric MgO-Al₂O₃spinel phase.
 2. The fused ceramic grainsaccording to claim 1, wherein the minimum MgO content, as a percentageby weight on the basis of the oxides, is 2.3%.
 3. The fused ceramicgrains according to claim 1, wherein the minimum MgO content, as apercentage by weight on the basis of the oxides, is 2.45%.
 4. The fusedceramic grains according to claim 1, wherein the maximum MgO content, asa percentage by weight on the basis of the oxides, is 4%.
 5. The fusedceramic grains according to claim 1, wherein the maximum MgO content, asa percentage by weight on the basis of the oxides, is 2.5%.
 6. The fusedceramic grains according to claim 1, wherein the maximum Na₂O content,as a percentage by weight on the basis of the oxides, is 0.1%,preferably 0.05%.
 7. The fused ceramic grains according to claim 1,wherein the mean size of said corundum crystals is between 18 and 20 μm.8. The fused ceramic grains according to claim 1, wherein 90% of saidcorundum crystals have a size of greater than 9 μm and 90% have a sizeof less than 27 μm.
 9. The fused ceramic grains according to claim 1,wherein 100% of said corundum crystals have a size of greater than 5 μm.10. The fused ceramic grains according to claim 1, having a grit numberof F60 or less according to FEPA Standard 42-GB-1984, and presenting thefollowing size distribution, with test sieves according to ASTM E11-87,Test Remainder Test Test Test sieves 3 in the sieve 1 sieve 2 sieve 3and 4 bottom pan aperture 425 300 250 212 size (μm) Residue 0 30 max 40min 65 min 3 max (%).


11. The fused ceramic grains according to claim 1, having a grit numberof F36 or less according to FEPA Standard 42-GB-1984, and presenting thefollowing size distribution, with rest sieves according to ASTM E11-87Test Remainder Test Test Test sieves 3 in the sieve 1 sieve 2 sieve 3and 4 bottom pan aperture 850 600 500 425 size (μm) Residue 0 25 max 45min 65 min 3 max (%).